1. Field of the Invention
This invention relates to a polysiloxane resin composition, specifically, a polysiloxane resin composition comprising first and second polysiloxanes, in which the second polysiloxane is composed of a cyclic or linear polysiloxane. The cured polysiloxane resin composition has good flexural strength and high reflectance properties.
2. Description of the Related Art
Optoelectronic devices, such as light emitting diodes (LEDs), are usually protected by a case or a package made from resin materials.
Polyphthalamide (PPA) is the typical resin that is used for packaging an optoelectronic device. However, physical properties of the resin are likely to deteriorate after a long period of usage. For example, the generated heat due to the operation of the optoelectronic device leads to photodegradation of the resin, which consequently leads to yellowing of the resin and decreased output power of the optoelectronic device. It occurs especially in high energy LEDs, such as UV LEDs, white light LEDs, and blue light LEDs. Thus, packaging materials with high reflectance and heat resistance are necessary.
US Application Publication No. 2010/0081748 discloses a silicone resin composition for optical semiconductor devices. The silicone resin composition is composed of (A) 100 parts by mass of an organopolysiloxane with an average compositional formula of (CH3)aSi(OR1)b(OH)cO(4-a-b-c)/2, (B) 3 to 200 parts by mass of a white pigment, (C) 400 to 1000 parts by mass of an inorganic filler, (D) 0.01 to 10 parts by mass of a condensation catalyst, and (E) 2 to 50 parts by mass of an organopolysiloxane having a linear diorganopolysiloxane moiety represented by the following formula,

wherein R13 and R14 are, independently of each other, a functional group selected from the group consisting of a hydroxyl group, an alkyl group having 1 to 3 carbon atoms, a cyclohexyl group, a vinyl group, a phenyl group and an alkyl group, wherein m is an integer ranging from 5 to 50.
By gel permeation chromatography (GPC) measurement with the polystyrene standard, the organosiloxane (A) has a weight average molecular weight, ranging from 500 to 20,000, preferably from 1,000 to 10,000, and more preferably from 2,000 to 8,000. Similarly, the average molecular weight of component (E), determined by GPC, is preferably from 3,000 to 1,000,000, more preferably, from 10,000 to 100,000.
As shown in Table 1 of the U.S. application, the bending strength of a specimen made from the aforesaid composition is inversely correlated with the amount of the linear organopolysiloxane (E). Noted, adding the linear organopolysiloxane with molecular weight over 3000 could lower the mechanical properties of silicone resin.